1. [Field of the Invention]
The present invention relates to a method for growing single crystals of a dissociative compound semiconductor useful for IC substrates of high electrical resistivity and optical device substrates with high dopant concentration
2. [Description of the Prior Art]
In known methods for growing single crystals of a dissociative compound semiconductor, a pulling process (referred to as the Czochralski method) can readily provide round wafers with a &lt;100&gt;direction and is advantageous in the production process of IC or optical devices. In the crystal growth of a dissociative substance, it is required to prevent the escape of a volatile component of the dissociative substance. Various methods which are roughly classified into two types have been employed for this requirement. A first method is the Liquid Encapsulated Czochralski method (hereinafter referred to as the LEC method) in which the surface of melt from which a single crystal is pulled is covered with a liquid sealant, such as B.sub.2 O.sub.3, and an inert gas pressure is applied onto the liquid sealant in order to suppress the escape of the volatile component. In a second method, the surface of the melt is covered with an atmosphere of a volatile component gas of a dissociative compound with a controlled pressure throughout the crystal growth operation. The first method has been industrially widely employed because of the simplicity of the apparatus.
However, the LEC method has presented difficulties in providing high quality crystals for the following reasons.
1. It is impossible to obtain a precisely controlled stoichiometry in the resulting crystal by control of evaporation pressure.
2. It is not easy to reduce the temperature gradient across the solid-liquid interface, because with low temperature gradient the escape of volatile component increases.
In order to practice the second method, as previously disclosed in U.S. Pat. application Ser. No. 644 840, filed Aug. 28, 1984 (corresponding to Japanese Patent Application Nos. 58-157 883 and 59-109 632), the present inventors have proposed an apparatus for pulling crystals in an atmosphere of a volatile component gas of a dissociative compound wherein an inner chamber contained in the crystal pulling apparatus is sealed and the pressure of a volatile component gas within the inner chamber is precisely controlled.
The practical process for pulling crystals using the apparatus proposed by our previous application set forth above is illustrated by referring FIG. 5. The inner chamber 1, 2 for sealing the volatile component gas of the dissociative compound is made of a material which is not subject to attack by the volatile component gas atmosphere and the chamber is constructed so as to be capable of being divided at its junction portions 3. At the junction portions 3, a liquid or solid sealant 7 is used and the divided chamber portions are pressed to each other by a driving shaft 13 to ensure tight sealing. A spring 8 as a stressrelaxation mechanism or alternatively a load cell as an automatic control mechanism of the pushing-up force, both of which are provided on the driving shaft, can keep the chamber free from any excessive stress due to thermal expansion, etc. Such a structural arrangement not only provides a satisfactory sealing performance of the chamber, but also make possible repeated use of the chamber. The use of liquid B.sub.2 O.sub.3 in rotating seals 15 permits a pulling shaft 5 and a crucible supporting shaft 14 to be moved vertically and rotated. The pressure of a volatile component gas within the sealed chamber is controlled by keeping the temperature of a pressure controlling furnace 10 not only constant but also lower than any other parts of the sealed chamber wall, and condensing the volatile component 11 at the projected part in the furnace 10. The growing crystal in the growth chamber can be observed through an optical transparent rod 9 during the pulling operation.
Such a specially designed apparatus, unlike any other known apparatus made of quartz, extremely facilitates the dividing and sealing operations of the chamber, thereby making possible not only repeated use of the chamber but also the production of large diameter crystals. The advantages have made the apparatus more useful in industrial applications. Incidentally, in the figure, other reference numerals are as follows:
4: crucible, 6: single crystal, 12: heater, 16: crucible pedestal, 17: melt, and 39: outer chamber of the whole apparatus.
However, the apparatus in FIG. 5 developed by the inventors to carry out the foregoing second method has been found to have the following disadvantages by further investigation, namely:
In the inner chamber 1 and 2, the temperature is highest at the crucible portion and is lower toward the upper portion. At the projected portion of the volatile component gas pressure controlling furnace 10, the temperature is lowest.
Such a temperature profile can be supposed to cause a violent convection in the gas within the inner chamber. Actually, fluctuations of image caused due to the changes in the density of the atmospheric gas within the chamber are observed through the optical transparent rod 9 after melting the source material. Such a convection means that the gas pressure in the chamber varies widely from place to place and is unstable with respect to time. This is unfavorable for the purpose of controlling precisely the stoichiometry under a constant gas pressure. Therefore, a further improvement is demanded in order to apply stably the pressure of the volatile component gas pressure controlling furnace 10 onto the surface of the melt without being subjected to any influence of the convection.
Further, the same problem also arises from a temperature profile of the melt contained in the crucible. In the pulling process, the crucible is located at a position slightly higher than the highest-temperature position in the inner chamber and the temperature in the melt is gradually reduced toward the upper portion. Such a temperature gradient will unavoidably cause convection in the melt.
A random flow of heat due to the convection will give rise to an irregular change in the temperature at the solid-liquid interface and is undesirable for the purpose of stoichiometric control under constant conditions. In addition, the thermal convection in the melt hinders a steady crystal growth and, microscopically, will introduce various crystal lattice defects into a grown crystal as a result of repetition of solidification and remelting.
In view of the above, it is highly desirable to suppress the foregoing convection in the gas of the volatile component of the dissociative compound in the sealed chamber and the convection in the melt within the crucible and thereby make it possible to pull under the conditions of constant pressure and constant temperature.